Species of the obligate anaerobe, Bacteroides occupy a prominent position in the microbial ecology of the human oral cavity and intestinal tract. Although normally indigenous, Bacteroides of oral origin are often associated with periodontopathic dental plaque. One intestinal species, B. fragilis, is the most commonly isolated anaerobic organism from soft-tissue infections. The emergence of resistance to the drug of choice, clindamycin (Ccr), and to other antibiotics threatens to compromise the treatment of Bacteroides infections. Although resistance phenotypes have been traced to plasmids, many strains possess transferable resistance elements that are not associated with plasmid DNA. In this application we shall test the hypothesis that the Ccr determinant of a B. fragilis plasmid, pBF4, is a transposon. These studies will involve base sequence analysis of the ends of this putative transposon and genetic tests of its ability to undergo serial transposition. Major effort will be directed towards defining the genetic basis of the non-plasmid associated Ccr in Bacteroides. Using physical and genetic approaches we shall test the hypothesis that one such element occupies a chromosomal integration site. We shall also explore the possibility that resistance transfer elements occur in oral Bacteroides and we shall assess their ability to exchange resistance determinants with intestinal species. This work will use a wide array of modern genetic methodology including mutant isolation, transmission genetics, molecular cloning, in situ filter blot hybridization and DNA base sequence analysis.